The present invention relates to the field of oversampled, noise-shaping signal processing which includes, for example, sigma-delta modulation technology. More specifically, the present invention provides methods and apparatus for improving the dynamic range and noise performance of an oversampled, noise-shaping modulator. Even more specifically, some embodiments of the present invention improve the performance of such modulators for the purpose of driving power switching devices.
In response to the inability of pulse width modulation (PWM) technology to provide both the dynamic range and the noise performance for a variety of high end and high power switching applications, several attempts have been made to design switching amplifiers using oversampled, noise-shaping modulators, specifically sigma-delta modulators, for their noise-shaping characteristics, see H. Ballan and M. Declercq, 12 V .SIGMA.-.DELTA. Class-D Amplifier in 5 V CMOS Technology, pp. 559-562 (IEEE 1995 Custom Integrated Circuit Conference), the entirety of which is incorporated herein by reference. However, as will be discussed, including power MOS transistors in a sigma-delta modulator loop presents additional problems which hinder overall amplifier performance. A standard first order sigma-delta modulator 100 is shown in FIG. 1. An integrator 102 is connected in series with a comparator 104 which is essentially a two-level quantizer with a sampling rate f.sub.s. The output of comparator 104 is fed back to integrator 102 via digital-to-analog converter (D/A) 106 and adder 108. The feedback forces the low frequency content of the quantized output signal to track the low frequency content of the input to modulator 100. Any difference between the quantized output and modulator input is accumulated in integrator 102 and eventually corrected. For first-order sigma-delta modulators, noise in the signal band due to quantization error is reduced by approximately 9 dB for each doubling of the oversampling ratio (OSR). The OSR is given by f.sub.s /2f.sub.o, where 2f.sub.o is the Nyquist rate, i.e., twice the bandwidth f.sub.o of the baseband signal. For second-order sigma-delta modulators, this noise is reduced by approximately 15 dB (9 dB+6 dB) for the same increase in OSR. For third-order modulators, the reduction is 21 dB. However, noise improvements achieved by increases in the OSR, i.e., increases in f.sub.s, are ultimately constrained as the rise and fall times of the output signal become significant with respect to the sample period. For a thorough discussion of sigma-delta modulation techniques, see Candy and Temes, Oversampling Delta-Sigma Data Converters, pp. 1-25 (IEEE Press, 1992), the entirety of which is incorporated herein by reference.
As mentioned above, the insertion of power MOS transistors in a standard sigma-delta modulator is accompanied by other performance problems. For example, in audio applications, power MOS transistors drive relatively low impedances and must therefore have output impedances smaller than one ohm for good overall efficiency. As a result, the switching characteristics of such transistors are relatively slow, varying from an ideal switching characteristic in an asymmetric way, and thereby generating distortion which is typically at or above the -60 dB level. Because standard sigma-delta modulators employ digital or state feedback (e.g., D/A 106 of FIG. 1), the asymmetric edges of the power transistor output are not seen by the integrator stages. Consequently, standard sigma-delta modulators are not able to correct for the distortion introduced by the power MOS transistors because of the exclusive use of state feedback.
Moreover, because modern sigma-delta modulators use sampled integrators, simply employing state feedback via a digital-to-analog converter to the integrator stages has not been effective. This is due to the fact that sampled integrators tend to have aliasing problems with high frequency distortion. In addition, the delay introduced by a power MOS transistor stage causes the feedback to be increasingly uncorrelated with the input, further undermining the feedback's corrective function. The additional delay due to a power MOS transistor stage can also adversely affect circuit stability. In short, any noise reduction improvements achieved by the use of standard sigma-delta modulation are rendered insignificant by the distortion introduced by the power MOS transistors and associated driver stages.
An improvement to standard sigma-delta technology which addresses these problems is described in commonly assigned, copending U.S. Pat. application Ser. No. 08/667,925 for METHOD AND APPARATUS FOR OVERSAMPLED, NOISE-SHAPING, MIXED-SIGNAL PROCESSING filed on Jun. 20, 1996, the entire specification of which is incorporated herein by reference. In that application an oversampled, noise-shaping modulator is described which employs continuous-time feedback from the output of its switching stage rather than pure state feedback from before or after the power switching stage. The continuous-time feedback is provided in such a way as to reduce the aliasing effects on the feedback path introduced by the switching stage which might otherwise interfere with the baseband to an unacceptable degree. That is, the improved modulator of the above-described application combines the use of continuous-time feedback to compensate for low frequency distortion, and some means of attenuating the aliasing effects of high frequency distortion introduced via the feedback path.
FIG. 2 is a simplified block diagram of an embodiment of a second-order modified oversampled, noise-shaping digital amplifier 200 designed according to a specific embodiment of the invention described in the above-referenced application. An input signal is introduced to a first continuous-time integrator stage 202 via adder 204. The output of first integrator stage 202 is transmitted to a second continuous-time integrator stage 206 via adder 208. A clocked comparator stage 210 sampled at sample frequency f.sub.s receives the output of second integrator stage 206 and transmits the resulting logic signal to power switching stage 212. The continuous-time output of power switching stage 212 is fed back to first integrator stage 202 via continuous-time gain stage 214 and adder 204. Continuous-time feedback is also provided to second integrator stage 206 via continuous-time gain stage 216 and adder 208. In this example, an anti-aliasing filter is not employed in the feedback path because the integrator stages are continuous-time integrators which inherently reject high frequencies. If, on the other hand, integrator stages 202 and 206 comprise sampled integrators, the feedback to gain stages 214 and 216 would be via an anti-aliasing filter. Such a filter would typically be a low pass filter which reduces the aliasing effects of the high frequency distortion generated by the power switching stage by removing the high frequency distortion from the continuous-time feedback signal. For additional details regarding continuous-time feedback with sampled integrators, please refer to the above-referenced copending patent application.
The modified sigma-delta technology described in the above-referenced application provides a highly efficient, low noise alternative to PWM in a wide variety of applications. However, because there is a limit on the amount of gain which can be introduced in the oversampled modulator loop, and therefore a limit on the dynamic range of the modulator, there are some applications for which even the excellent noise performance of the modulator of the above-referenced application may need improvement. For example, in the field of power amplifiers, a very low noise floor and a high dynamic range with low distortion are obviously desirable. In other power switching technologies (e.g., regulators, motor drivers, etc.) other performance improvements (e.g., reduced ripple and component size) are desirable. However, as mentioned above, significant improvement in these parameters generally requires increasing the gain in the oversampled modulator loop. The modulator loop gain, in turn, is limited, at least in part, by the modulator's sampling frequency. Unfortunately, increasing the sampling frequency, i.e., increasing the oversampling ratio, to improve the dynamic range is ultimately of limited utility in that power switching applications have some minimum pulse width below which the resulting distortion would quickly overcome any improvements in dynamic range. That is, power switching applications typically employ large power devices which swing very nearly from power supply rail to power supply rail. In addition, the speed at which these power devices can switch is limited by their typically large parasitic components. Therefore, because the switching of these power devices is characterized by very large and relatively slow transitions, the input pulses to the power switching stage must be sufficiently long and sufficiently far apart to ensure that the transitions do not overlap to an unacceptable degree. For further information regarding the characteristics of power devices, please refer to Power MOSFETS: Theory and Application by Duncan A. Grant and John Gowar (.COPYRGT.1989 John Wiley & Sons, Inc.), and Smart Power ICs: Technologies and Applications edited by B. Murari, F. Bertotti, and G. A. Vignola (.COPYRGT.1996 Springer Verlag).
The following example should be illustrative. Through the use of an oversampled modulator, the noise floor of a power amplifier may be improved from 9 to 21 dB (or more) per octave of oversampling depending on the order of the modulator, i.e., 9 dB for a first-order modulator to 21 dB for a third-order modulator (see the references cited above and the above-referenced patent application). On average, today's power MOSFETs can typically be switched up to around 1.5 MHz. Thus, a designer of an audio power amplifier might set the sampling frequency of the oversampled modulator at 3 MHz, providing 64.times.oversampling (i.e., 26) with respect to an audio input signal. Assuming a third-order modulator, the corresponding theoretical input-referred noise floor, i.e., the dynamic range, of the amplifier would be given by (21 dB).times.(log.sub.2 64) or 126 dB. Actual third-order modulator performance is typically 20-30 dB worse than this due to practical considerations. While this level of performance might be suitable for some low end applications, it is not sufficient for a high-end, high-power audio amplifier as the high gain associated with such an amplifier will boost the input-referred noise to unacceptable levels. Increasing the sample frequency to 6 MHz yields an oversampling ratio of 128.times.which, in turn, provides a theoretical noise floor/dynamic range of (21 dB).times.(log.sub.2 128) or 147 dB. While this is sufficient for many high end applications, the sampling frequency is too fast for today's power devices
One possible alternative to increasing the sampling frequency is to make the oversampled modulator a fourth-order modulator to obtain a theoretical dynamic range of (27 dB).times.(log.sub.2 64) or 162 dB. Unfortunately, there are at least two practical problems with this approach. First, it is already a difficult problem to stabilize a third-order modulator because of the delay introduced by the power switching stage. It is even more difficult to stabilize a fourth-order modulator. Second, even though the dynamic range would theoretically increase with a fourth-order modulator, the output swing which could be achieved would be reduced. For a discussion of this phenomenon, please refer to T. Ritoniemi, T. Karema, and H. Tenhunen, Design of Stable High Order 1-bit Sigma-Delta Modulators (IEEE Proc. ISCAS '90, pp.3267-3270, May 1990), the entirety of which is incorporated herein by reference. This output swing reduction is unacceptable in power switching applications because the output signal of a power amplifier must typically be able to go from rail to rail, or at least very close.
It is therefore apparent that there is a need for a way in which the noise floor/dynamic range of a power switching amplifier may be improved beyond the level currently attainable with oversampled, noise-shaping modulators.